1. Field of the Invention
The present invention relates to a method of evaporating thin film and, more particularly, to a method of evaporating thin film used in an organic electro-luminescent (EL) display.
2. Description of the Related Art
In new generation panel display technologies, organic electro-luminescent (EL) display using organic compounds as luminescent materials is a laminated-film type display and has advantages of self luminescence, thin profile, light weight, and low driving voltage. In general, on a glass substrate of the OLED, a lamination body comprises an anode layer, a hole-injecting layer, a hole-transporting layer, an organic luminescent film, an electron-injecting layer, an electron-transporting layer, and a cathode layer. After applying an outer voltage to the OLED, both the electrons generated from the cathode layer and the holes generated from the anode layer move to reach the organic luminescent film, and then bombard the film and combine to transform electricity into luminosity.
In fabricating the lamination body, conventional photolithography is not suited for patterning because the EL elements are weak with respect to water and oxygen. Thus evaporation with a shadow mask is used to deposit and pattern thin film on the glass substrate at the same time. As shown in FIG. 1, a metal mask 12 with a plurality of openings 11 is placed below a glass substrate 10, and an evaporation source 14 is placed below the metal mask 12. During evaporation, the materials in the evaporation source 14 can pass through the openings 11 to be deposited on the predetermined regions of the glass substrate 10. However, the evaporation source 14 is a single/point source that causes a shadow effect. Also, the materials piling in the evaporation source 14 are not compact enough to reduce the thermal conductivity, thus a uniform temperature is not easily achieved in the evaporation source 14, resulting in poor uniformity of deposition thickness. One solution is to reduce the distance between the evaporation source 14 and the glass substrate 10, and the other is to rotate the glass substrate 10 during evaporation. Both methods may, however, decrease evaporation efficiency.
U.S. Published Application No. 2001/0006827 discloses a method of using linear evaporation source to form the lamination body. As shown in FIGS. 2A and 2B, a shadow mask 18 with a plurality of openings 17 is placed below a glass substrate 16, and a linear evaporation source 20 with a plurality of evaporation cells 22 arranged in a line is placed along X direction below the shadow mask 18. During evaporation, the linear evaporation source 20 is moved along Y direction, and the materials in each of the evaporation cell 22 can pass through the openings 17 to be deposited on predetermined regions of the glass substrate 16. However, this generates high costs and a complex process for controlling increased parameters. Also, this only improves the deposited-thickness uniformity in direction Y, but not direction X. In addition, this still cannot solve the above-mentioned problems of difficulty in achieving an uniform temperature during evaporation, and low efficiency in evaporation source material use.